Research On VSA-HVDC System Modeling And Small Disturbance Stability

With the promotion of the capability and technological advances in power semiconductor switching devices,HVDC based on voltage source converter has been a rapid development.At present,with the large-scale development and utilization of new energy,the capacity and voltage level of VSC-HVDC project constantly improve.VSC-HVDC operation stability has caused wide public concern by power grid company and the scholars.In this background,the paper focuses on the steady state mathematical model and power flow calculation method of VSC-HVDC system,establishes the dynamic mathematical model of VSC-HVDC system,puts forward a kind of VSC-HVDC system small signal stability analysis method.Converter valve equivalent model is established based on the switching function,then a dynamic simulation system is designed.First,a node impedance matrix based Gauss-Seidel(GS)method was presented for DC grid steady state power flow calculation.Test system has proved and verified the convergence advantages and computational efficiencies of the nodal impedance matrix based GS method than the nodal admittance matrix based GS method and Newton-Raphson(NR)method.Steady state power flow algorithm model of AC/DC hybrid power system has established in this paper.Alternate AC/DC iteration method was also introduced for the power flow calculation,and the response of DC power station when the power changed in the inverter,the influence on the network are also verified and checked by various operation modes.Using sensitivity analysis method to determine the optimal installation location of DC power flow controller(DCPFC)in VSC-HVDC power grid.Introducing the method of equivalent power injection,the new bus caused by the installation of the DCPFC are transferred to both of the ending buses of the branch in the form of additional injection power,preventing the increase of order number and change of element in node admittance matrix and the jacobin matrix.Second,dynamic model of VSC-HVDC has established in dq synchronous rotation coordinate system.The dynamic model include the dynamic models of DC network,AC network and interface network of VSC-HVDC,and constant power control convertor station,constant DC voltage convertor station.In order to make the model is suitable for the future DC grid coordinate control request,the droop control dynamic model in detail was established in this paper.Using PSCAD/EMTDC the five terminal VSC-HVDC system model was established,then the validity of dynamic model are verified by time domain simulation.To compare the dynamic response with the traditional master-slave control,further evidence that droop control station can actively participate in the power regulation,prevent the main converter station overload effectively.Third,this article put forward the VSC-HVDC system small signal stability study is divided into two parts: namely VSC-HVDC dc side stability analysis and ac/dc interconnected stability analysis.The stability analysis of the dc side is through the establishment of VSC-HVDC complex-frequency domain simplified circuit of VSC-HVDC,and then to get the equivalent loop gain for judging the stability of the VSC-HVDC and setting parameter via adopting ESAC criterion.It wouldn't need to know the detailed parameters of a given converter station in actual application,so it is suitable for different converter valve topology of VSC-HVDC system.For the stability between VSC-HVDC and AC network,the paper firstly discusses the stability principle,builds the equivalent circuit of VSC in dq reference frame of the current loop control structure diagram based on dq synchronous rotation coordinate system,and then deduces the equivalent admittance expressions of converter station.Secondly,the ac impedance is tested through the uncharacteristic harmonics current injection method,and using frequency domain characteristic of ac/dc impedance ratio to judge stability,then the stable control parameters domain are given;Finally,a time domain electromagnetic transient simulation of the VSC-HVDC system shows that the different ac impedance influences on stability of the VSC-HVDC system,which further verifies the proposed stability criterion analysis and control parameter setting method are correct and effect for the stability analysis of the VSC-HVDC system.Finally,the article propose design MMC dynamic simulation system based on equal time constant.The MMC converter valve quivalent RLC model was established based on switching function.According to similarity theories,the paper presents a determining method of the modular multilevel converter(MMC)similarity criterion and derives the similarity criterion of the MMC.And then,the electrical circuit law and equivalent time constant based similarity criterions of the MMC are proposed and used as the necessary fundamental theories and design principles of the MMC physic analogue simulation system.The results in engineering application proved the rationality of dynamic simulation system.Based on the small signal stability analysis method,the parameters are set.According to passive inverter and STATCOM experiment of MMC-HVDC dynamic model,the comparisons of MMC-HVDC real-time digital simulation system with physic analog simulation were done and showed the accuracy of the MMC real-time modelling.The mathematical model and digital simulation model of MMC HVDC further improved in this paper.